Byzantium has some really powerful units early in the game, and a near-inpenetrable fortress (Constantinople). The problems they face are Crusades, which easily avoided if you pay a "tithe" to the Pope in Rome (why cant there be an Anti-Pope for Orthodox factions?), Constantinople is a primary target for the Mongols, and that their effectiveness drops to almost nil once everyone else gets gunpowder. Not the hardest faction if you blitzkrieg the map.

Scotland, while lacking in gunpowder, has early pikemen and the best position in the game. Once you kick out the English, the entire islands of the UK and Ireland are now yours. The AI is barely able to make amphibious assaults, giving you a fortress without walls. The fact that Venice will most likely beat you to any Crusade objective is not all that good, however.

My vote? The Turks or Russians. Poor early units, and extreme proximity to the Mongols and Tiramids. For the Turks, buiilding up Georgia is vital to keeping the Mongols out of Turkey. For Russia, the land is so spread out that the Mongols will eventually find their way to your capital. Russia is also in close proximity to Poland and Hungary.